A number of different types of reactors and storage towers for cellulose of medium and/or low consistency are known. The low consistency area, typically at 3-8% pulp consistency, is a much easier application area, since the pulp substantially behaves like a fluid and can more easily be pumped to and from these towers and reactors without a risk for blockage. The medium consistency area, typically at 8-16% pulp consistency, is a much more difficult application area where it is necessary to consider plugging and stoppage of the feeding. Cellulose pulp of 10% consistency can due to the holding together by the fiber network be held in the hand and only after squeezing can the liquid be pressed out. At this consistency the pulp can easily form plugs in inlets and outlets if those are not shaped in the right way.
In, for example, U.S. Pat. No. 5,319,902 a method is shown for converting a down-flow tower, of a mass tower type, from one application for storage of low consistency pulp to an application for storage of medium and/or high consistency pulp. Here a lower bottleneck portion of the mass tower is filled with a filler material (cement or similar material) in which a discharge pipe is mounted in the built-up concave bottom surface. By this design the bottom of the tower is converted so that it obtains a shape that is adapted for a smooth discharge of medium or high consistency pulp without the risk of blockade in the discharge pipe. The solution in U.S. Pat. No. 5,319,902 requires a relatively expensive conversion though where a tower with a more or less permanent curved bottom is obtained. If the tower is to be converted to a low consistency tower of a mass type it is necessary to chisel away the filler of the bottom construction.
In an up-flow tower that holds medium consistency pulp it is necessary to make sure that the flow established through the cross-section of the tower/reactor is always of a similar flow across the whole cross-section so that so called canal formations are not established. Canal formations establish a central flow through the reactor that only uses a fraction of the total volume of the tower/reactor.
With the purpose of establishing an even flow across the cross-section the bottom of the tower may have many inlets so that the inlets evenly distribute the flow across the whole cross-section. An example of such as variant is shown in U.S. Pat. No. 3,992,248 where these inlets are sequentially opened in a predetermined order. An other alternative is a reactor inlet that Kvaerner Pulping AB has sold for decades where a rotating cylinder that extends into the bottom of the reactor has a radially directed inlet in the mantel surface of the cylinder wall so that the pulp flow is radially dispersed over the bottom of the tower under the rotation of the inlet.
Another alternative in order to avoid these distribution arrangements is to form the inlet part of the up-flow tower so that it is conical with a cone angle α relative the horizontal plane in the order of 30-80°, preferably 45-60°. An example of such an up-flow reactor that has a conical inlet is shown in U.S. Pat. No. 5,034,095. Through this conical inlet part the flow from the inlet will be successively distributed across the cross-section. The drawback of these towers is that they are virtually impossible to use a down-flow tower for medium consistency pulp since the conical part provides a substantial plug forming effect for medium consistency pulp that flows down the conical part. If the technique in, for example, U.S. Pat. No. 5,319,902 is used to convert such up-flow towers with a cone shaped inlet it would be necessary to fill almost the entire volume of the conical part with cement. This would reduce the storage capacity of the tower and the tower could not easily be converted to an up-flow tower if so is desired without having to chisel away the filler material of the tower which is a time consuming and expensive procedure.
In U.S. Pat. No. 5,538,597 a solution is shown where cellulose pulp with a higher consistency is withdrawn from a second highly placed outlet opening from a mass tower so that the pulp is diluted to a low consistency at the lower dilution zone of the tower. In the main variants a short inserting tube is used for the second/upper outlet that has such a length that the stationary boundary layer is only passed at the walls of the tower and reaches the formed flow. In this way, pulp of medium consistency can be withdrawn. In a variant shown in FIG. 8 a vertically directed insertion tube was used that according to the directions is inserted into the upper part of the tower i.e. substantially above the dilution zone of the lower part of the tower which according to the figure is shown so that the end of the insertion tube is above the level of the conical conversion part of the mass tower. The purpose of this solution is to provide a possibility to also withdraw pulp of medium consistency while simultaneously withdrawing pulp of low consistency. The formed flow passed the outlets of the pipes is a requirement to be able to pump out medium consistency pulp also (the end of the pipe must reach into “ . . . said zone of moving pulp . . . ”, see claim 1). By using this solution only a fraction of the volume (and dwell time) of the tower can be used as storage or treatment tower for medium consistency pulp since the main part of the pulp passes the upper pipe and is later discharged as low consistency pulp and where the volume over the insertion pipe is only a smaller portion of the upper part of the tower.
In U.S. Pat. No. 6,371,526 a solution is shown for handling pulp flows of medium consistency in pipes where at the flow transition from large diameter pipes to pipes with smaller diameters a special quick cone is used where the pipe with the small cross-section is inserted into the larger flow cross-section. There is a conical wall connection from the wall of the bigger pipe to the wall of the smaller pipe at the same level as seen in the flow direction after the end of the small pipe. In this way a simpler quick cone (transition from the thicker pipe to thinner pipe) is obtained for medium consistency pulp without the risk for plugging. This solution is mainly intended for simplified quick cones for flows from bigger pipes to pipes with a smaller flow cross-section.